In recent years, research and development have been extensively conducted on light-emitting elements utilizing electroluminescence (EL). In the basic structure of such a light-emitting element, a layer containing a light-emitting substance is interposed between a pair of electrodes. By voltage application to this element, light emission from the light-emitting substance can be obtained.
Such light-emitting elements are self-luminous elements and hence have advantages over liquid crystal displays in having high pixel visibility and eliminating the need for backlights, for example; thus, light-emitting elements are thought to be suitable for flat panel display elements. Light-emitting elements are also highly advantageous in that they can be thin and lightweight. Furthermore, very high speed response is also one of the features of such elements.
Furthermore, since such light-emitting elements can be formed in a film form, they make it possible to provide planar light emission; thus, large-area elements can be easily formed. This is a difficult feature to obtain with point light sources typified by incandescent lamps and LEDs or linear light sources typified by fluorescent lamps. Thus, light-emitting elements also have great potential as planar light sources applicable to lighting devices and the like.
Such light-emitting elements utilizing EL can be broadly classified according to whether the light-emitting substance is an organic compound or an inorganic compound. In the case of an organic EL element in which a layer containing an organic compound used as the light-emitting substance is provided between a pair of electrodes, application of a voltage to the light-emitting element causes injection of electrons from the cathode and holes from the anode into the layer containing the organic compound having a light-emitting property and thus a current flows. The injected electrons and holes then lead the organic compound having a light-emitting property to its excited state, so that light emission is obtained from the excited organic compound having a light-emitting property.
An excited state formed by an organic compound can be a singlet excited state or a triplet excited state. Luminescence from a singlet excited state (S*) is called fluorescence, and luminescence from a triplet excited state (T*) is called phosphorescence. In addition, the ratio of S* to T* formed in a light-emitting element is statistically considered to be 1:3.
At room temperature, observations of a compound that can convert energy of a singlet excited state into luminescence (hereinafter, referred to as a fluorescent compound) usually show only luminescence from the singlet excited state (fluorescence) without luminescence from the triplet excited state (phosphorescence). Thus, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element using a fluorescent compound is assumed to have a theoretical limit of 25% based on a S*-to-T* ratio of 1:3.
In contrast, with a compound that can convert energy of a triplet excited state into luminescence (hereinafter, called a phosphorescent compound), luminescence from the triplet excited state (phosphorescence) is observed. Further, with a phosphorescent compound, since intersystem crossing (i.e., transition from a singlet excited state to a triplet excited state) easily occurs, the internal quantum efficiency can be increased to 75% to 100% in theory. In other words, an element using a phosphorescent compound can have three to four times as high emission efficiency as that of an element using a fluorescent compound. For these reasons, a light-emitting element using a phosphorescent compound has been actively developed in recent years in order to achieve a highly-efficient light-emitting element.
When formed using the above-described phosphorescent compound, a light-emitting layer of a light-emitting element is often formed such that a phosphorescent compound is dispersed in a matrix of another compound in order to suppress concentration quenching or quenching due to triplet-triplet annihilation in the phosphorescent compound. Here, the compound serving as a matrix is called a host material, and the compound dispersed in a matrix, such as a phosphorescent compound, is called a guest material.
A host material needs to have higher triplet excitation energy (an energy difference between a ground state and a triplet excited state) than a phosphorescent compound in the case where the phosphorescent compound is a guest material.
Furthermore, since singlet excitation energy (an energy difference between a ground state and a singlet excited state) is higher than triplet excitation energy, a substance that has high triplet excitation energy also has high singlet excitation energy. Therefore the above substance that has high triplet excitation energy is also effective in a light-emitting element using a fluorescent compound as a light-emitting substance.
In Patent Document 1, 3-(4-biphenylyl)-5-(4-tert-butylphenyl)-4-phenyl-1,2,4-triazole (abbreviation: TAZ) is used as a host material for a phosphorescent compound that emits green light.